The long-term goal of the PI's research is to understand neural mechanisms of sound localization in vertebrates, including humans. The primary goal of this proposal is to determine how the primitive authority system of fish functions in sound localization. The PI's previous studies and others have demonstrated that fish determine the axis at which a sound wave is propagating by using arrays of spatially oriented sensory hair cells in the ear. However, it is not known how the brain processes the peripherally coded directional information in order to extract the specific direction of the sound. The proposed work will elucidate mechanisms underlying central auditory processing of directional information. It has been proposed that the fish ear is stimulated by a sound wave through two distinct pathways: direct particle motion input and indirect pressure input via the swim bladder. It was hypothesized that fish (with a swim bladder) first determine the axis on which the sound is propagating and then determine its direction by comparing the timings of inputs carrying pressure and particle motion information within the brain. The PI will test the two steps of this hypothesis on two functionally distinguishable teleost fishes, the sleeper goby and goldfish. Specific aims address anatomical organization and functional processing of directional information in the medulla and midbrain: 1) Auditory nuclei responsible for directional coding. Experiments will be carried out to reveal central projection sites of the saccular, lagenar, and utricular nerves and medullary projection sites in the midbrain using both anterograde and retrograde labeling methods. 2) 3D structure and cytoarchitecture of the auditory nuclei. Combined with the neuronal tracing, boundaries of the auditory nuclei will be defined to create their three dimensional structures using Neurolucida. Golgi stain studies will characterize types, sizes, and orientations of the auditory neurons. 3) Brain representation of the peripheral map of directional coding. Using whole-cell filling of Neurobiotin and confocal imaging, a 3D map of peripheral directional coding will be reconstructed to reveal how the saccular map of directional coding is represented in the medulla. 4) Directional response properties of auditory medullary and midbrain neurons. Single-cell recording and filling will characterize directional response properties, locations, and morphologies of auditory medullary and midbrain neurons to form a neural map(s) of best response axes of the neurons. 5) Differential-phase sensitive neurons. Single-cell recording will determine if there are neurons in the medulla and/or midbrain of the goldfish that encode specific timing differences between particle motion and pressure inputs